The present invention relates to a hydrometallurgical process to recover nickel and cobalt from nickeliferous laterite ores and, in particular, to the atmospheric leaching of both limonite and saprolite ores with a mineral acid to dissolve nickel and cobalt wherein iron is rejected as jarosite.
The known reserves of nickel and cobalt in nickeliferous oxide ores, e.g., those referred to as laterites comprising limonite and saprolite ore, are far greater than the corresponding reserves in sulfide ores. An important disadvantage when processing laterite ores, however, is the inability to beneficiate these ores by conventional techniques.
A number of new hydrometallurgical processes are being developed for the extraction of nickel and cobalt from nickeliferous laterite ores. Many of these processes require the dissolution of the metal values with sulfuric acid at high temperature (245.degree.-270.degree. C.) and pressure (525-785 psig), followed by solid-liquid separation and neutralization of residual free acid present at ambient pressure. This is the basic "Moa Bay Process", as described by J. R. Boldt and P. E. Queneau in "The Winning of Nickel", Methuen, London, 1967. In this process, the nickeliferous ore is first made into a pulp having a solids content of about 40% before leaching at high temperature and pressure. During pressure leaching most metals dissolve and iron and aluminum are rejected by hydrolysis to hematite and alunite, respectively. After leaching, the pulp is cooled and washed by counter current decantation and the solids are directed to tailing treatment. Excess acid is neutralized and the remaining iron and aluminum are precipitated as hydroxides with the addition of coral mud. Nickel and cobalt are subsequently recovered via sulfide precipitation.
Several variations of the high-pressure acid leach (HPAL) method have been devised with the aim of improving the process and economical aspects. For example, U.S. Pat. No. 4,044,096 provides guidelines to optimize the high-pressure acid leaching of nickeliferous lateritic ores through a combination of operational steps to improve the economics and efficiency of leaching. The steps include scalping laterite ore to remove the coarse (high magnesium) fraction and thus lower the acid consumption.
The HPAL process is most amenable for high iron ores containing 40 wt % iron or higher. Lateritic ores with an iron content less than 40 wt % contain in general a higher amount of acid consuming magnesium and are therefore not economically suitable for direct high pressure leaching. U.S. Pat. No. 3,804,613 teaches a method of high-pressure acid leaching of saprolite ore at relatively low acid/ore ratios by preconditioning the saprolite with leach liquor from the high-pressure leach step. No mention is made of concurrent limonite leaching.
U.S. Pat. No. 3,991,159 teaches the use of saprolite ore to neutralize acid resulting from the high-pressure acid leach of limonite ore. Leaching of the saprolite fraction is carried out at high temperature (1500.degree.-250.degree. C.) and pressure for effective iron and aluminum rejection, but with relatively low nickel extraction from the saprolite ore. In another process, U.S. Pat. No. 4,097,575 teaches saprolite ore roasting at 500.degree.-750.degree. C. under oxidizing conditions to increase its neutralization capacity before neutralization of HPAL liquors. This process suffers from the high capital cost needed for roasting facilities.
While the prior art HPAL methods obtain a high extraction of nickel and cobalt, they require the use of expensive equipment and sophisticated materials of construction to withstand the use of concentrated acid at the high temperatures needed (200.degree.-300.degree. C.). Furthermore, part of the rejected iron and aluminum are in the form of hydroxides, which are difficult to deal with. Several alternatives to the HPAL process to recover nickel and cobalt from laterite ore have been proposed.
For example, U.S. Pat. No. 4,062,924 describes a method for leaching limonite ores in acidic media at temperatures up to 110.degree. C. and in the presence of hydrogen sulfide gas to precipitate dissolved nickel and cobalt. Most dissolved iron is also reduced to the divalent oxidation state however, consuming very high amounts of the reducing gas in addition to high acid consumption. U.S. Pat. No. 4,065,542 teaches a similar method. In this process, ferrous iron produced by the method described above is used to leach metal values from manganiferous sea nodules. U.S. Pat. No. 4,511,540 illustrates a way to recover nickel and cobalt from ores with a manganiferous matrix by leaching with sulfuric acid in the presence of sulfur dioxide gas at temperatures below the boiling point of the liquid solution. None of these processes includes the treatment of saprolitic ores.
In the process of U.S. Pat. No. 3,793,432, limonite ore is leached with sulfuric acid at a pH below 1.5, while simultaneously adding alkaline iron-precipitating agents. The process is carried out at atmospheric pressures, but requires leaching times in excess of 40 hours and usually from 60 to 100 hours for efficient nickel extraction and iron precipitation. No use of saprolite is made in this process. U.S. Pat. No. 4,410,498 teaches a method to leach saprolite ore with sulfuric acid at atmospheric pressure, while adding a reducing agent to maintain the redox potential between 400 and 600 mV. In another process, described in U.S. Pat. No. 5,571,308, nickel and cobalt are leached from saprolite ore by contact with a mineral acid at room temperature or in the temperature range of 60.degree.-80.degree. C. The leaching mode can be conducted by heap, vat, or agitation leaching.
We have now found an efficient and economical method to leach both limonite and saprolite ores in a single process at atmospheric pressure, to obtain high percent dissolution of nickel and cobalt and reject iron from the leach liquor as a jarosite compound at the same time. The method avoids the high capital costs associated with sophisticated autoclaves.